This invention relates to nuclear reactor fuel assemblies and in particular to assemblies which are mounted side by side in a reactor core.
The fuel rods of each assembly are held by spacer grids between an upper end fitting or top nozzle and a lower end fitting or bottom nozzle. The reactor coolant flows upwardly from holes in the lower end fitting along the fuel rods, and upwardly through holes in the upper end fitting.
When the fuel assembly is loaded in a reactor core, an upper core plate over the fuel assembly reacts against fuel assembly holddown spring members on the upper end fitting, to provide a downward force. This force combines with the fuel assembly weight to prevent fuel assembly liftoff from hydraulic forces during operation of the reactor pumps.
The holddown spring members are exposed to the high pressure and temperature of the circulating coolant. They experience flexure while accommodating relative movement between the fuel assemblies and the support plate, and they experience some friction at the active surface which is in contact with the core support plate or when stacked with each other. This friction can have two adverse consequences. First, the spring coefficient can be affected if the point of contact of the spring active surface against the support plate or each other, cannot adjust as the spring flexes. Secondly, and perhaps more importantly, friction can produce wear of the upper core plate and spring which can affect the available spring force and the integrity of the spring member itself.